This invention relates generally to a method and apparatus for forming Bragg refractive index gratings in a waveguide, and more particularly to a method for forming such gratings using Lloyd's mirror, together with a phase delay plate for shifting the location of the grating away from the interface between the mirror and the waveguide for producing a longer, preferably symmetrical grating.
Optical waveguides, both fiber waveguides and planar waveguides have become increasingly important for a number of applications, including communications. A number of devices for providing specific functions in waveguides are best performed by waveguides that include optical waveguide gratings, more particularly refractive index gratings within the core of the waveguide. One approach to making optical waveguide gratings is to form an interference pattern from two interfering beams derived from a single frequency laser, which, if sufficiently intense, write a refractive index grating into the waveguide core over a period of time. The formation of a Bragg grating in this manner is known.
Bragg gratings can be formed that are either persistent or transient, that is gratings which are substantially permanently formed in the core and which persist after the grating forming radiation is removed, or gratings which persist only as long as the grating forming radiation is incident on the core, as described for example, by Faco et al. in U.S. Pat. No. 5,218,651.
In order to form a high quality grating, long coherence length optical sources have been required to reduce the formation of side lobes in the loss spectrum of filters created in the fiber. Long coherence sources are available, such as dye lasers and the like, but they are expensive, not particularly rugged, difficult to tune and keep in tune, and are generally unsatisfactory for use in a manufacturing environment.
Excimer lasers, which are superior for manufacturing operations, by virtue of their cost, ruggedness, stability and the like and which are suitable for writing gratings when used with a phase mask, have a shorter coherence length than dye lasers and have not been satisfactory for forming Bragg gratings in waveguides for this reason.
It is an object of this invention to provide a method and apparatus for forming a refractive index grating in a waveguide that overcomes the disadvantages of known methods and apparatus, and particularly a method and apparatus that allows the use of short coherence length optical sources in a modified Lloyd's mirror method that is particularly well suited to manufacturing environments.
Briefly stated, and in accordance with a presently preferred embodiment of the invention a method for forming a refractive index grating in a waveguide includes the steps of positioning the waveguide against tile edge of a mirror, directing a first portion of a coherent beam of optical radiation on the mirror at a first angle, and a second portion of the coherent beam of optical radiation on an optical phase delay plate, and combining the first and second portions in an interfering beam and directing the interfering beam on the waveguide to form a grating in the waveguide.
In accordance with another embodiment of the invention, the optical phase delay plate comprises a silica plate for delaying the second portion of the beam by an amount sufficient to shift the interference pattern away from the edge of tile mirror.
In accordance with still another aspect of the invention, the interference pattern is shifted sufficiently away from the edge of the mirror to form a symmetrical grating in the waveguide.
Apparatus for forming a refractive index grating in a waveguide in accordance with the invention includes a waveguide formed from a material that changes its index of refraction when exposed to a beam of optical radiation, a source of coherent optical radiation for forming a beam of radiation a mirror arranged to intercept a first part of a beam and to reflect the first part of tile beam through an angle, and a phase delay plate arranged to intercept a second part of the beam for delaying the second part of the beam propagating through the plate in which the mirror and the phase delay plate are arranged so that the first part of the beam and the second delayed part of the beam form an interference pattern on the waveguide for changing the index of refraction of the waveguide, in a pattern corresponding to the interference pattern for forming an index grating in the waveguide.
In accordance with another aspect of the invention, the mirror and the phase delay plate are arranged for forming a symmetrical pattern in the waveguide having its center shifted away from the line of intersection of the waveguide and the mirror.